Mucin glycoprotein domains are important components of the cell surface landscape prevalent on various cell types including those of the immune system. These are characterized by a high level of O-glycosyalted serine and threonine in their sequences. Two proteins with such mucin domains, CD43 and CD45, comprise about 30% of the surface of T-cells. Such domains are significant in cell-cell interactions and recognition, and molecular localization related to these events have consequences for the signal transduction events mediated by their intracellular domains. Both carbohydrate and polypeptide components of the glycoproteins contribute to molecular recognition, with O-glycosylation having a profound conformational effect, inducing an extended arrangement of the protein backbone. The significance of the carbohydrate epitopes on these glycoproteins is born out by their specific variation with the state of the cell, stage of development and disease, offering an additional dimension to the diversity for cellular signaling and a target for cell specific therapeutic intervention. The extended structure of CD43 makes it one of the first cellular components encountered by the outside environment. Since molecular recognition is central to the roles of these giycoproteins, the proposed research will investigate their structure, required to understand function and interactions. These molecules have resisted detailed structural analysis in large part because of difficulties in isolating suitable homogeneous material from natural sources. Initially, our studies have demonstrated that chemical synthesis and nuclear magnetic resonance spectroscopy techniques can be joined to obtain a high resolution description of a mucin motif from CD43. Our aims are to extend this, first, to determine structures of other representative motifs and further elucidate the principles controlling molecular organization, second, to use this in characterizing larger segments of mucins, and third to relate this to their recognition by other molecules such as antibodies and galectin. Interactions of CD43 and CD45 with galectin-1 causes their segregation into separate patches and ultimately T-cell programmed cell death. Our studies will examine this interaction, providing insight into this process and into possible control of autoimmune disease. On tumor cells, these glycoproteins are characterized by their aberrant glycosylation, and a structural understanding could aid in further development of glycopeptide anti-tumor vaccines.